Conventionally, an oscillator has been utilized for providing timing (providing synchronization) for operations of circuits, in an electronic apparatus or the like. In such a case, the oscillator has been used as a device for outputting an electric signal (reference clock signal) which forms a reference for operations of circuits. In view of a stable operation of the electronic apparatus, such an oscillator has been desired to output a high-quality reference clock signal having a less phase noise.
Further, in a communication apparatus, an oscillator has been utilized as a device for creating a reference signal for signal transmission. Along with increased demands for speed-up of wireless communication and for sophistication of global positioning system (GPS), in recent years, the oscillator has been required to further improve the phase stability of an oscillation signal (to further reduce the phase noise).
An oscillator using a quartz crystal unit (quartz oscillator) is one example of current oscillators. However, such a quartz crystal unit has various problems that it is hard to reduce in size and is unsuitable for integration and, further, it involves larger numbers of trial production processes, a poor yield, and longer delivery times.
In recent years, attention has been focused on an oscillator employing, as a resonator, a micro electro-mechanical system (MEMS) formed from silicon or the like through a semiconductor process, as an oscillator capable of overcoming various problems of a quartz crystal unit.
FIG. 17 is a diagram illustrating a basic structure of a conventional MEMS oscillator.
A conventional MEMS oscillator 999 is a feedback-type oscillation circuit which includes a MEMS resonator 1001 as a resonance circuit, and a driver amplifier 1002 as an amplification circuit and, further, is adapted to feed back and amplify an output (an oscillation signal) from the driver amplifier 1002 through the MEMS resonator 1001, with a closed loop 1003.
The MEMS resonator 1001 includes an input electrode 1001a, a MEMS oscillating member (MEMS vibrator) 1001b and an output electrode 1001c, wherein the input electrode 1001a and the oscillating member 1001b are spaced apart from each other, and the oscillating member 1001b and the output electrode 1001c are spaced apart from each other, with a predetermined interval interposed therebetween. Further, a bias voltage (a DC voltage, VDC) is applied to the MEMS oscillating member 1001b. 
If an oscillation signal (an input signal) 1004 (AC voltage, VAC) is inputted to the input electrode 1001a, an excitation force is exerted on the MEMS oscillating member 1001b. Further, if the frequency of the oscillation signal 1004 (VAC) is coincident with the mechanical resonance frequency of the MEMS oscillating member 1001b, this causes the MEMS oscillating member 1001b to largely oscillate, which causes the output electrode 1001c to output a feed-back signal 1006 (a displacement electric current).
FIG. 18 is a graph illustrating changes of resonance characteristic due to changes of the signal level of the oscillation signal 1004 (the amplitude of VAC) which is inputted to the MEMS resonator 1001. A characteristic 1011 represents a resonance characteristic of the MEMS resonator 1001, in a case where the signal level of the oscillation signal 1004 falls within a predetermined proper range. In cases where the signal level of the oscillation signal 1004 falls within the predetermined proper range as described above, the resonance characteristic of the MEMS resonator 1001 exhibits bilateral symmetry centered at the resonance frequency fc. In this case, within the predetermined proper range regarding the signal level of the oscillation signal 1004, in oscillating the MEMS oscillating member 1001b, the oscillation signal 1004 has such a signal level as to prevent prominent appearances of the effects of pulling the MEMS oscillating member 1001b by the input and output electrodes (1001a and 1001c) (capacitive bifurcation), due to excessive electrostatic forces exerted between the MEMS oscillating member 1001b and the input and output electrodes (1001a and 1001c) as the MEMS oscillating member 1001b gets too close to the input and output electrodes (1001a and 1001c). Further, the maximum value within the proper range is a value proportional to the product of the Q factor of the MEMS resonator 1001, the signal level of the inputted oscillation signal 1004 (the amplitude of the voltage VAC), and the bias voltage (VDC) applied to the MEMS oscillating member 1001b and, thus, can be preliminarily determined. In this case, the Q factor, which is a dimensionless number obtained by dividing the resonance frequency by a half width, is an index indicating the acuteness of the resonance, which can be used in evaluating the performance of the resonator.
As indicated by an arrow 1014, if the signal level of the oscillation signal 1004 inputted to the MEMS oscillator 1001 is increased beyond the proper range, distortions appear in the resonance characteristic of the MEMS resonator 1001 as in characteristics 1012 and 1013, which degrades its bilateral symmetry, thereby causing decreases of the peak gain and deterioration of the Q factor. Further, if an oscillation signal 1004 at a further increased level is inputted to the MEMS resonator 1001, this may collapse the MEMS resonator 1001. Namely, the MEMS resonator 1001 has the characteristics of degrading the peak gain and the Q factor and, also, degrading the stability of the resonance frequency, if the voltage of the signal inputted thereto is excessively larger. Therefore, it is desired to control the driver amplifier 1002 such that the signal level of the oscillation signal 1004 inputted to the MEMS resonator 1001 falls within the proper range.
FIG. 19 is a block diagram illustrating the structure of a MEMS oscillator described in Patent Literature 1 (JP 2004-201320 A).
A MEMS oscillator 1000 in Patent Literature 1 includes a MEMS resonator 1001, a driver amplifier 1002, and an auto gain control unit (Auto Gain Control, AGC) 1005. The MEMS oscillator 1000 forms a closed loop 1003, similarly to the MEMS oscillator 999. Further, the AGC 1005 is adapted to monitor the amplitude of an output from the driver amplifier 1002 (an oscillation signal 1004) and to control the gain of the driver amplifier 1002.
In the MEMS oscillator 1000, the AGC 1005 controls the gain of the driver amplifier 1002, such that the level (the amplitude) of the oscillation signal 1004 (the AC voltage) falls within the proper range. By doing this, it is possible to prevent the AC voltage 1004 at an excessively-higher level from being inputted to the MEMS resonator 1001. In this case, the proper range refers to a range of the level of the AC voltage 1004 which prevents occurrence of distortions in the resonance characteristic of the MEMS resonator 1001 as described above.
As described above, in Patent Literature 1, it is possible to prevent deterioration of the resonance characteristic of the MEMS resonator 1001 due to inputting of the AC voltage (the oscillation signal 1004) at an excessively-higher level to the MEMS resonator 1001, thereby preventing increases of phase noises contained in the oscillation signal 1004.